MRI Lets Scientists See Inside Batteries

It's easy to tell when a battery is dead. But engineers who want to build better batteries can't see inside them to study the finer points of chemistry without cutting them open and ruining them. Now there's a better way.

Magnetic resonance imaging (MRI) scanners, commonly used to examine our brains, are now being used to study our batteries.

Battery technology is increasingly important as engineers try to devise better ones to power our cars, phones, and pretty much everything else. That's why Clare Grey, a professor of chemistry at Cambridge University, teamed up with NYU chemist Alexej Jerschow to develop a way of imaging batteries through MRI.

At first, MRI doesn't seem like an ideal solution for working with batteries. It uses a combination of magnetic fields and radio waves, so it's notoriously unkind to metal implants, hearing aids, and jewelry, literally ripping them from their owners. A battery with a metal casing wouldn't fare much better in an MRI scanner—plus, the radio waves used in MRI don't penetrate metal well. ("If you were to listen to the radio inside a metal box, the reception would be terrible" Grey says.)

So Grey and Jerschow's built a tiny lithium battery, put it in a plastic shell, and stuck it in a carefully calibrated MRI scanner. Up to now, the only way to analyze a battery's insides was to cut it open, which ruins it for future use. But this technique lets them see what happens to a battery's insides at different points in its life.

Dealing With Dendrites

In this MRI study, the scientists took scans of the battery when it was first created and after it was charged. They found a significant chemical shift after the lithium was charged. When charged, the metal became less concentrated, and growths began to form.

A graphic representation of the differences in dendrite formation on the pristine test battery (left) and the same battery after charging (right). Credit: Grey et. al. / Nature Materials

These moss-like crystalline growths, called dendrites, are the bane of battery builders. They form at the barrier between the anode and cathode (- and + sign respectively on any battery). "The lithium dendrites will grow through the separator in the same way that ivy grows through concrete," Grey says. Dendrites tend to form in lithium batteries when the temperature increases dramatically or a charge is introduced too quickly; they can interrupt the charge and can cause batteries to short circuit. "If you charge too fast then you get these dendrites and degradation byproducts, and everything can go horribly wrong," Grey says.

These pesky formations are the reasons that most lithium batteries on the market today are lithium-ion rather than lithium-metal. Lithium-ion batteries contain a lithium compound (lithium combined with other materials like cobalt, iron, phosphate, or others) and are rechargeable. They can develop dendrites if not recharged properly, severely curtailing battery performance.

Lithium-metal batteries, on the other hand, aren't rechargeable, but they can hold large amounts of energy for long periods of time, which is why they are used in watch batteries. However, they are highly reactive (think of the high school science experiments in which lithium reacts violently with water) and they form dendrites much more easily than lithium-ion batteries. Previous attempts to create rechargeable lithium metal batteries ultimately failed because dendrites grew so rapidly during the charging process that they caused explosions and fires.

"We would like to have more lithium-metal batteries," Nancy Dudley, a battery expert from Oak Ridge National Laboratory says. If scientists can figure out under what conditions it can be used reliably, batteries could become thinner and more powerful—not just for electronics, but also for use in cars and renewable energy. But until scientists understand the growth patterns of dendrites, purely lithium-metal batteries will remain rare.

Better Batteries

Ideally, Grey and her colleagues would be able to see inside commercial batteries with typical metal casings, the technology to see through metal isn't quite there yet. By using plastic-encased cells, the scientists can see the same process of dendrite formation at work without the interference.

And while MRI promises to be an easy way for researchers to explore what goes on in a battery, it isn't the only one. Dudley's colleagues at Oak Ridge have probed batteries with neutron beams, examining the layers of a battery inside the case, looking for internal changes that a vehicle battery goes through over the course of many charging and discharging cycles. She adds that there are other less high-tech ways of probing a battery for defects. Thermal imaging is frequently used to check batteries for hot spots that could indicate a problem in the system. Even X-rays can give researchers a snapshot of the structure inside a battery, showing where lithium ions are in relation to the other parts of the battery.

So far, MRI's offer the best look inside the structure, but there is still a ways to go before the method is perfect. The images produced right now can see the general outline of the dendritic growths but nothing more specific. "The resolution isn't as good as it possibly could be," Grey says.